Mountain Home Demonstration State Forest contains one of the southernmost concentrations of giant sequoias, and sits in a region where two major geological forces are in tension: the ancient, massive Sierra Nevada block on one side, and the actively extending Basin and Range Province on the other. Normal faults cross this landscape, and the evidence of crustal stretching — fault scarps, tilted blocks, and springs along fault zones — is visible throughout the grove.
Key Facts
Tectonic settingSierra Nevada / Basin & Range boundary
Fault typeNormal faults (extensional)
Extension rate~1 cm/year (Basin & Range)
Extension onset~30 million years ago
Key featureFault scarps, slickensides, fault springs
Elevation~1,800–2,200 m
The Basin and Range Province
The Basin and Range Province is a region of the western United States characterised by a series of north-trending mountain ranges separated by broad, flat valleys. It extends from southeastern California and Nevada through Arizona and into Mexico. The province is formed by crustal extension — the lithosphere is being stretched and thinned, causing the crust to crack along normal faults and blocks to drop down between them.
The Sierra Nevada forms the western boundary of the Basin and Range Province. As the Basin and Range extends eastward, the Sierra Nevada block is being pulled apart from the ranges to its east. This extension began roughly 30 million years ago and is continuing today at a rate of about 1 centimetre per year across the full width of the province.
Normal Faults at Mountain Home
Normal faults are fractures in the Earth's crust along which one block has moved downward relative to the other, accommodating crustal extension. At Mountain Home, several normal fault systems trend roughly north-south, consistent with the east-west extension direction of the Basin and Range. Where these faults intersect the surface, they create fault scarps — small cliffs that can be a few metres to tens of metres high, depending on the cumulative offset.
Fault scarps are geologically young features. In the Sierra Nevada's semi-arid climate, scarps are slowly eroded by slope wash and creep, but fresh scarps from recent earthquakes can be identified by their sharp morphology. Older scarps have rounded, subdued profiles. Geologists use the morphology of fault scarps to estimate their age and the frequency of past earthquakes on a given fault.
Slickensides: Evidence of Fault Motion
On fault surfaces exposed in outcrops at Mountain Home, you can sometimes find slickensides — polished, grooved rock surfaces formed by friction as the two sides of a fault slide past each other. The grooves, called slickenlines, record the direction of fault movement. Their orientation can tell geologists whether movement was purely dip-slip (up and down) or had a component of strike-slip (sideways) motion.
Minerals precipitated on fault surfaces — typically calcite, quartz, or chlorite — can also provide information. In some cases, these minerals can be dated, providing direct evidence for the age of fault activity. At Mountain Home, fault-zone mineralogy reflects the hydrothermal fluid circulation that is common along active fault systems, bringing warm mineral-laden water from depth.
Isostatic Rebound and Uplift
The Sierra Nevada has been rising — uplifting — for millions of years, driven partly by isostasy. Isostasy is the principle that the crust floats on the mantle like an iceberg on water: remove weight from the surface (by erosion) and the crust rises; add weight (by ice sheets) and it sinks. As erosion has stripped millions of metres of rock from the Sierra Nevada since the batholith was emplaced, the range has isostatically rebounded upward.
At Mountain Home, the combination of isostatic uplift and Basin and Range extension has created a complexly faulted landscape where ancient granitic rocks are juxtaposed against younger, fault-bounded blocks. Springs are common along fault zones, where fault-zone clay minerals create permeability barriers that force groundwater to the surface. These springs have historically supported the sequoias at Mountain Home, providing water even during dry years.
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